Digital watermarking is a process for modifying physical or electronic media to embed a machine-readable code into the media. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media signals such as images, audio signals, and video signals. However, it may also be applied to other types of media objects, including text documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
Digital watermarking systems typically have two primary components: an encoder that embeds the watermark in a host media signal, and a decoder that detects and automatically reads the embedded watermark from a signal suspected of containing a watermark (a suspect signal). The encoder embeds a watermark by altering the host media signal. The reading component analyzes a suspect signal to detect whether a watermark is present. In applications where the watermark encodes information, the reader extracts this information from the detected watermark.
Several particular watermarking techniques have been developed. The reader is presumed to be familiar with the literature in this field. Particular techniques for embedding and detecting imperceptible watermarks in media signals are detailed in the assignee's U.S. Pat. Nos. 6,122,403 and 6,614,914, which are hereby incorporated by reference.
The invention provides methods for embedding digital watermarks for authentication of printed objects, and corresponding methods for authenticating these objects. One aspect of the invention is a method of embedding a digital watermark in a digital image to be printed on an object. The method embeds an auxiliary signal in the digital image so that the auxiliary signal is substantially imperceptible, yet machine readable. It converts the image to a halftone image using an inherently unstable halftone screen structure that is likely to cause ink flow errors when reproduced. The errors introduced by reproducing the unstable halftone screen structure incorrectly are automatically detectable by reading the auxiliary signal.
Another aspect of the invention is a method of authenticating a printed object. The method receives a digital image scanned of the printed object, and detects a digital watermark in the digital image. It then uses a measurement of strength of the digital watermark to detect reproduction errors due to inaccurate reproduction of unstable halftone screen structures in the printed object. Another aspect of the invention is a method of authenticating a printed object using different watermarks in the object. This method detects a first digital watermark from a substrate of the printed object, and detects a second digital watermark from an image scanned from the printed object. It uses the relationship between the first and second digital watermarks to determine authenticity of the printed object.
Another aspect of the invention is a method of authenticating a printed object using the geometric relationship between visible fiducials and embedded watermarks on a printed object. This method detects a visible fiducial in an image scanned from the printed object, and detects a location of a digital watermark hidden in the printed object. It determines authenticity of the printed object by comparing the location of the digital watermark to the visible fiducial. Further features will become apparent with reference to the following detailed description and accompanying drawings.